1. Field of the Invention
The present invention relates, in general, to a method of manufacturing a photoreceiver, and, more particularly, to a method of simultaneously manufacturing a high electron mobility transistor (HEMT) and a waveguide integrated metal-semiconductor-metal photodetector (MSM PD) used in a photoreceiver.
2. Description of the Related Art
While optical communication is beginning and is becoming widely supplied, thorough attempts are made to monolithically integrate a transistor and a photodiode on an InP substrate. Of various monolithic integration methods, a method of monolithically integrating an HBT (Hetero-junction Bipolar Transistor) and a p-i-n photodetector by sharing the base and the collector layer of HBT is regarded to be excellent in view of cost and efficiency. However, in an HEMT part of a thin film layer is not shared to be monolithically integrated, as in the HBT. If the thin film structure of the HEMT is changed into a shared structure to realize monolithic integration, the HEMT may have simpler processes and better surface evenness than the HBT, and thus, the HEMT monolithic integration method comes to be a better technique than the integration methods using the HBT and the p-i-n photodetector. The p-i-n photodetector is a photodiode having a structure composed of a p-region, and an n-region, and an intrinsic (i type) semiconductor layer between the p-region and the n-region.
In the HEMT structure, since a buffer functions to capture electrons in a quantum well using the potential difference of a conduction band, it is formed mainly of a larger band gap material. However, since most electrons in the HEMT are confined only in the ground state and the first excited state of the quantum well, it is good for the buffer with narrow bandgap materials if the conduction band offset of materials is high enough for constructing discrete energy levels up to the first excited state in the quantum well. That is, even if the material, which has a band gap small enough to absorb light at a wavelength of 1.3 μm for optical communication, is used for the buffer of the HEMT the conduction band minimum of this buffer is higher than the first excited state of the quantum well, therefore the electrical properties of the HEMT are not greatly changed. In addition, in InP-based semiconductors, the smaller the band gap it is, the larger the refractive index it has. Hence, as seen in FIG. 3, an optical waveguide 118 composed of a small band gap buffer, a substrate and air may be realized.